1. Field of the Invention
The present invention relates to a zoom lens unit, an imaging device and a photographing device.
2. Description of Related Art
Such a zoom lens unit, which includes in order from an object side: a first lens group having a positive refracting power; a second lens group having a negative refracting power; and a subsequent lens group comprising one or more lens groups and having a positive refracting power as a whole, and an aperture stop disposed between the second lens group and the subsequent lens group, wherein when changing a modification from a wide angle end to a telephoto end, a distance between the first lens group and the second lens group increases, and a distance between the second lens group and the subsequent lens group decreases, is suitable for “miniaturization and high magnification ratio”, and is embodied widely for video camera and digital camera as the zoom lens unit therefor.
The zoom lens unit with such a structure is most common for “a zoom lens unit for digital camera” with a magnification ratio more than 4 times, as well as the zoom lens unit for the video camera having the magnification ratio of 10 times or more as a matter of course. In such a zoom lens unit, a total of 6 or more than 6 lenses are often used in “the first and the second lens groups located nearer to the object side than the aperture stop”, due to the necessity of the aberration correction.
In a case that the photographing is taken in a state so-called “state of backlit” that a strong light source such as the sun exists in or neighbourhood at the photographing screen, a ghost image formed by round trip lights due to reflection between two lens surfaces reaching the image plane, is generated depending on the lens structure. Since in general the zoom lens unit is constructed by the large number of lenses, measures against the ghost image are an important technological issue.
Especially, reflection between lens surface to lens surface which are nearer to the object side than the aperture stop, reaches the image plane to generate the ghost image easily, and the measures against the ghost image are extremely important in the zoom lens unit like above-mentioned, in which the number of the lens forming the first and the second lens groups is comparatively increased.
Recently, there are many users requiring the wider field angle of zoom lens unit. For the zoom lens unit with structures mentioned above, a half field angle of 38 degrees or more at the wide angle end is also proposed, and thus it becomes easy that the light causing the ghost image appears in a screen, simply because the field angle is wide.
Moreover, when the field angle of the zoom lens unit like above-mentioned is widened, the diameters of the first and the second lens groups tend to increase easily, and even in the case that the light source causing the ghost image exists outside of the screen, the generation probability of the ghost image caused by the reflection between the lens surface to lens surface of the first and the second lens group is high. In the case that the strong light source exists in the photographing screen, the ghost image is easily tolerated because it is not so distinguished, as compared with an image of the strong light source, while in the case that the light source is outside of the photographing screen, the ghost image is difficult to be tolerated since it is apt to be distinguished.
Measures against the ghost image for the zoom lens unit are disclosed in JP-A-2001-324676, JP-2991554 B.
The zoom lens unit disclosed in JP-A-2001-324676 is a zoom lens unit like above mentioned, its first lens group includes a bonded surface, and reduction of the ghost image is obtained by inhibiting the reflection on the bonded surface. However, as an embodiment, only a zoom lens unit with a half field angle at a wide angle end being smaller than 34 degrees is described specifically, and the inhibition of the ghost image in a case that the half field angle at the wide angle end is 38 degrees or more is uncertain.
In JP-2991554 B, there has been disclosed to apply a reflection inhibiting coat having reflection characteristics of mutually complementary relation, on two or more reflection boundary surfaces causing the ghost image, so as to prevent the generation of the ghost image in wide wavelength range. However, there is no specific data about the zoom lens unit itself, and the magnification ratio and the half field angle at the wide angle end are uncertain.